A slow moving, high torque engine or generator is known that operates on a very small temperature differential. This engine is commonly referred to as a Minto Wheel after its inventor Wallace Minto. The engine is arranged as a wheel that contains a series of sealed containers. The sealed containers are placed around the rim of the wheel and are aligned as diametrically opposed pairs. Each diametrically opposed pair is in fluid connection through a tube. The wheel rotates in a vertical plane. In any given pair at any given moment in time during the rotation, one of the containers is moving in a generally upward direction, and the other container is moving in a generally downward direction. At one position in the rotation, the containers are aligned vertically, with one container at the top being in the uppermost position and one container at the bottom being in the lowermost position. Each container moves between the uppermost and lowermost positions.
Each opposed pair of containers and the associated connecting tube form a sealed unit. Into each sealed unit a volume of a low-boiling liquid, for example propane, butane, carbon dioxide or Freon is introduced. For a given pair located at or near the vertically aligned position, most of the introduced volume of liquid is disposed in the lowermost container. The lowermost container is then exposed to a very mild increase in temperature, for example an increase of as little as 2° centigrade or about 3.5° F. The small temperature increase in the liquid in the lowermost container vaporizes a portion of the liquid, producing a higher pressure on the surface of the liquid. This pressure forces the liquid up the connecting tube and into the uppermost container. This transfer of liquid from the lowermost container to the uppermost container transfers mass to the uppermost container, causing the container to increase in weight while the lowermost container decreases on weight. Gravity pulls the uppermost container downward, turning the wheel in a manner similar to the turning of a water wheel. This rotational motion can be used for any desired mechanical work. Wheels of modest size can perform such tasks as pumping water for irrigation, grinding food grains and generating small amounts of machine power. The wheel turns relatively slowly, but produces enormous torque. This high torque rotational motion can be geared up to produce any speed desired at the final output shaft. Although output can be converted to higher speeds, the wheel or engine is most effective for applications that utilize high torque at low speed.
The horsepower produced by the rotating wheel is proportional to the product of torque and speed, i.e. revolutions per minute of the wheel. For a given wheel exposed to a given temperature difference between opposed containers, a particular maximum horsepower is produced when fully loaded. This maximum horsepower, i.e. the power output, of the wheel is proportional to the rate of heat transfer. Therefore, arrangements of an engine or generator that are not constrained by the rate of heat transfer into and out of a liquid are desired.